This invention relates to differential amplifiers, and more particularly to a distortionless differential amplifier employing bipolar transistors.
When an amplifier is formed by a bipolar transistor, the following relationship (1) exists between the collector current I.sub.C and the base-emitter voltage V.sub.BE : ##EQU1## where I.sub.S is the reverse saturation current, q is the electron charge, K is the Boltzman constant, and T is the junction absolute temperature. Thus, the input-output characteristic of the bipolar transistor is non-linear, and the collector output current waveform is significantly distorted with respect to the input voltage waveform.
In order to eliminate this distortion, a technique has been employed called "current negative feedback" in which an emitter resistor is inserted in the circuit. However, this method of distortion reduction is not desirable in that the distortion cannot be completely eliminated, the circuit gain is decreased, and the circuit becomes unstable if the amount of feedback is increased.
FIG. 1 is a circuit diagram showing one example of a differential amplifier which is generally employed. The emitters of a pair of differential PNP transistors Q.sub.3 and Q.sub.4 are connected together through current feedback emitter resistors R.sub.3 and R.sub.4, and bias currents are applied to the transistors by a current source I.sub.O. A reference potential, e.g. ground, is applied to the base of the transistor Q.sub.4, while an input signal e.sub.i is applied to the base of the transistor Q.sub.3. Differential inversion outputs are provided across the collector resistors R.sub.1 and R.sub.2 of the two transistors.
Let us consider the relationship of AC components only, which are obtained by ignoring the DC bias voltages and currents in the differential amplifier. If the AC components of the base-emitter voltages of the transistors Q.sub.3 and Q.sub.4 are represented by v.sub.be3 and v.sub.be4, respectively, then the relationship between the input voltage e.sub.i and the emitter currents i.sub.e3 and i.sub.e4 is: EQU e.sub.i =v.sub.be3 +R.sub.3 i.sub.e3 +R.sub.4 i.sub.e4 -V.sub.be4 ( 2)
The relationship between the voltage v.sub.ee between the two emitters and the positive phase output v.sub.O2 is: ##EQU2##
If the input signal e.sub.i changes in the positive direction (or in the increasing direction), the emitter current (substantially equal to the collector current) of the transistor Q.sub.3 is decreased, while the emitter current (substantially equal to the collector current) of the transistor Q.sub.4 is increased. Therefore, as is apparent from the characteristic curve of the expression (1) (not illustrated since it is well known in the art), the relationship between the variation width .DELTA.V.sub.BE3 and .DELTA.V.sub.BE4 of the base-emitter voltages of the transistors is: EQU .vertline..DELTA.V.sub.BE3 .vertline.&gt;.vertline..DELTA.V.sub.BE4 .vertline.(4)
Accordingly, in the expression (2), EQU v.sub.be3 -v.sub.be4 &gt;0 (5)
Therefore, as the input voltage e.sub.i is increased, in the expression (2) the rate of increase of the voltage v.sub.ee between both emitters (=R.sub.3 i.sub.e1 +R.sub.4 i.sub.e2) is decreased, and accordingly from the expression (3) the output voltage v.sub.O is also decreased. When the input voltage e.sub.i changes in the negative direction, the same phenomenon occurs. Therefore, the output waveform v.sub.O with respect to the sinusoidal input waveform e.sub.i is such that, as shown in FIG. 2, the positive and negative peaks are collapsed, thus producing distortion.